Wednesday, April 17, 2013

How does Atomic Fluorescence Spectroscopy measure up?

Whether it is fluorescent lighting, dental
fillings, antique switches, gold mining or thermometers, the element mercury
(Hg) is present in the world we live. Many
of the mercury containing products give us comfort, are used to provide us with
information, and even allow us to control our environment. While these products are safe, they could
potentially expose people to a plethora of toxic compounds if an accident
should occur. Symptoms of mercury exposure include seizures,
memory loss, and in some cases, death. Because
of these risks, several guidelines and regulations have been developed that
limit the amount of mercury people can be exposed to, and special methods are
required for cleaning up mercury if an accident should occur. Currently the time weighted average limit for
mercury varies depending on regulating agency.
For OSHA, the limit is 0.1mg/m3*; NIOSH sets the limit at
0.05mg/m3*; and the ACGIH has a limit of 0.025mg/m3*. Since mercury vapor is not something people
can see, how do they determine their amount of exposure? Arizona Instrument, LLC manufactures the J505
Atomic Fluorescence Analyzer; a handheld atomic fluorescence spectrophotometer
that measures the concentration of mercury in air. The lower detection limit of this instrument
is 50ng/m3 (0.000050mg/m3), and it can detect as high as 0.5mg/m3. These detection limits exceed the current
industrial exposure limits, as well as clean-up levels for public facilities.Atomic Fluorescence
Spectroscopy (AFS)

When an atom is
excited by an input of energy, one of its electrons transitions from a stable
ground state to an unstable excited state.
Once the source of energy is removed, the electron returns to its ground
state and the absorbed energy is emitted as a photon (light). This process is called fluorescence. Often the amount of energy given off is not
the same as the energy going in. This is
not the case for mercury, which makes it special. When the energy required to excite an
electron is the same energy as the photon it gives off when it returns to its
ground state, it is called resonance fluorescence, and is easily detectable
using AFS. The J505 instrument uses a mercury
lamp to excite the mercury atoms at the 254nm wavelength, and then uses a
detector to measure the emission of the photons, at the same wavelength, as the
electrons return to their stable ground states. Because AFS measures the
emission of photons, this technique does not have interferences, such as hydrocarbons,
hydrogen sulfide, and ammonia, which are often problematic for traditional
detection methods. The specifications
for the J505 Atomic Fluorescence Analyzer are below. Atomic Fluorescence Spectroscopy should not be confused
with Atomic Absorption Spectroscopy (AAS). In AAS, a light source of known wavelength and
intensity is passed through a sample of interest. Some of the energy of the
source light is absorbed by the sample as it energizes electrons in the
material from the ground state to an excited state. A detector is placed at the
end of the pathway to determine how much of the energy passed through. The
difference between the energy of the source light and the energy of the light
that arrives at the detector is directly proportional to the concentration of analyte
in the sample. One of the drawbacks of
this technique is that there are a number of other common molecules that can
absorb energy at the same wavelength as mercury. To compensate for these unwanted absorptions,
manufacturers use a variety of filtering techniques to limit background
interference. While these filtration principles are sound, they come at the
cost of a more complicated and bulkier instrument. Further, AAS can also
have physical limitations that may limit low level sensitivity. At very low concentrations, the
amount of absorbed light, when compared to the intensity
of the incident light source, can become indistinguishable from electronic
noise, making detection at these levels more challenging.

* These TWA
averages are dependent on time. For more
information on exposure limits please
visit each respective website.

1 comment:

I would like to thank you for your wonderful post in explaining in deep about the gas levels and maintain the atomic fluorescence. I was trying to learn about the gas analyzers and I ordered one from the best Gas Analyzers Hyderabad.

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